The invention relates to the field of microelectronics, and more specifically to the sector of the fabrication of microcomponents, especially those intended to be used in radiofrequency applications. It relates more particularly to microcomponents such as microinductors or microtransformers. It also relates to a process for fabricating such microcomponents, making it possible to obtain components having a high inductance and minimal resistive and magnetic losses.
As is well known, the electronic circuits used in radiofrequency applications include oscillating circuits formed by the association of a capacitor and an inductor.
The trend toward the miniaturization of appliances such as, in particular, portable telephones requires such components to be produced with an increasingly small size.
Moreover, these inductive components are required to have optimum electrical properties at increasingly higher frequencies, and over increasingly wide frequency ranges.
Thus, with regard to the Q-factor which characterizes the inductors, one problem that arises is that of parasitic capacitances existing between the turns forming an inductive coil.
Furthermore, for reasons of autonomy and of electrical consumption, it is also important to limit the electrical resistance of these inductors, which resistance also has an influence on the value of the Q-factor.
Thus, the invention proposes to solve several problems, namely the influence of the resistance on the value of the Q-factor of an inductor as well as the limitation in the self-inductance coefficient, imposed by the existing geometries.
Moreover, in radiofrequency applications, signal or current microtransformers are also used which have to meet the same size constraints as those identified in the case of inductors.
Furthermore, the problem arises of obtaining as perfect magnetic coupling as possible between the two windings of a transformer.
It has already been proposed to produce microcomponents which include inductive coils produced by micromachining techniques. Such surface-mounted microcomponents are produced by winding a copper wire around a ferrite core or a core made of a ferromagnetic material, followed by joining to contact pads on the outside of bars.
Microtransformers have also been produced using the same techniques, with additional problems inherent in putting them into a plastic package. Such components are very difficult to miniaturize which means that the possibility of reducing their electrical consumption is limited and they remain large in size, limiting their uses in portable appliances.
Moreover, it has already been proposed, as illustrated in document U.S. Pat. No. 5,279,988, to fabricate microinductors or microtransformers by means of technologies of the type used in microelectronics.
Nevertheless, these techniques involve processes having a large number of steps, which makes them complex, and indeed expensive. Furthermore, the concatenation of this multitude of steps does not allow optimum coupling between the turns of the coil and the magnetic core to be obtained.
Moreover, the solutions involving micromechanical processes prove to be ineffective, since the necessary tolerances in these technologies greatly limit the precision of such microcomponents.
The object of the invention therefore is to solve the problems of the size of microinductors or microtransformers, while maintaining very good electrical properties either in terms of the value of the inductance or the Q-factor, or in terms of magnetic coupling.
Another problem that the invention aims to solve is that of the complexity of the processes for fabricating such microcomponents.
The invention therefore relates especially to a process for fabricating an electrical microcomponent, such as a microinductor or microtransformer, which includes at least one coil and comprises a substrate layer.
This process comprises the following steps, consisting:
in etching a plurality of channels in the substrate, which channels are placed in an ordered manner as a band and are oriented so as to be approximately perpendicular to said band;
in electrolytically depositing copper in said channels so as to form a plurality of segments;
in planarizing the upper face of the substrate and of the plurality of segments;
in depositing, on top of said substrate and of said segments, at least one layer intended to form a core;
in etching the core in order for it to be preserved only above said band;
in depositing a plurality of arches on top of the core in a single electrolysis step, each arch connecting one end of a segment with one end of an adjacent segment, passing above said core.
Thus, the substrate serves as a mechanical support, stiffening the base of the component. Furthermore, when the substrate used has good dielectric properties, the parasitic capacitance between the various segments forming the base of the microcomponent is relatively low.
Thus, according to the invention, these microcomponents comprise turns in three dimensions, of approximately helical shape approaching as close as possible the ideal shape, namely, for inductors, of circular cross section which, per turn produced, has the least perimeter.
In order to produce microtransformers, the top part of the turns is made in the manner of a bridge which straddles the core that will serve as magnetic circuit.
In order to produce inductors, an operation to remove said core is furthermore carried out after the step of depositing the arches, the sacrificial core then being made of a soluble resin or organic polymer material.
Consequently, a microinductor in the form of a solenoid is obtained which has no material interposed between the turns except for that part of the substrate into which the bottom of the turns is anchored. In this way, a microinductor with a high self inductance is obtained, the inter-turn parasitic capacitance of which is extremely low.
Such inductors therefore operate within wide frequency ranges with a high Q-factor.
The use of copper, preferably with a thickness of a few tens of micrometers, furthermore makes it possible to greatly reduce the resistance of the coil and to greatly increase the Q-factor, right from the low frequencies.
In one embodiment, the core is made of a ferromagnetic material. This ensures that there is magnetic coupling between the various turns of the coil. Thus, if a microinductor produced, the use of a magnetic core further increases the value of the self inductance.
Moreover, if the magnetic core has a loop geometry, it is thus possible to produce microtransformers by making a second coil similar to the first, by selecting the ratio of the number of turns between these two coils depending on the desired application.
In practice, in order to produce components which include a magnetic core, an insulating layer is deposited after the planarization step but before the layer intended to form the magnetic core is deposited. After the core has been etched, an insulating layer is deposited on top of the core. In this way, the segments forming the bottom of the turns and the arches forming the top of the turns are not in contact with the magnetic material.
Nevertheless, the small thickness of these insulating layers allows optimum coupling to be obtained since the segments and the arches of each turn are as close as possible to the magnetic core.
Furthermore, when the component is intended to be used in a wet, or indeed chemically aggressive, atmosphere, a passivation layer is deposited on top of the arches. In this way, risks of copper corrosion, which would degrade the electrical properties, and especially the electrical resistance of such a component, are overcome.
As already stated, the invention relates not only to the fabrication process but also to the electrical microcomponents, of the microinductor or microtransformer type, which include at least one inductive coil and comprise a substrate layer.
These microcomponents are distinguished in that said coil is formed from a plurality of adjacent turns placed in series as a band, each of the turns consisting:
of a copper segment formed inside channels etched in the substrate;
of an arch connecting one end of said segment to one end of the segment of the adjacent turn, passing above said band.
Consequently, the coil of such a microcomponent is in the form of a solenoid of great strength since it is firmly anchored into a substrate layer and, moreover, having optimum electrical properties because of the monolithic bridge or arch shape of the upper part of the turns.
Thus, according to various embodiments, the microcomponent may include a core made of ferromagnetic material, passing through the turns and placed between the segments and the arches.
If the core forms a closed loop, the microcomponent may also include a second coil wound around said core, so as to form the microtransformer.
In the case of an inductor, the magnetic core is in the form of a bar.
According to one characteristic of the invention, the space lying between the arches of the adjacent turns is filled with air, thereby very greatly limiting the value of the inter-turn parasitic capacitance and allowing the use of such a microinductor at high frequencies.
In a preferred form, at least the arches are covered with a passivation layer made of a material chosen from the group containing gold and gold-based alloys.